This invention relates to a method and apparatus for measuring a light signal by frequency modulation, and more particularly to a method and apparatus for synchronously measuring the light signal in a spectrophotometer covering the ultraviolet and visible or the infrared wavelength range by frequency-discriminating the signal modulated to a certain frequency and demodulating the modulated signal in synchronism with the modulating operation.
In order to modulate the light signal from a cell, spectrophotometers are provided with a device such as a rotating chopper for alternately intercepting and passing the light beam from the cell. If the light beam to be chopped has such a large sectional area that cannot be neglected as compared with the mechanical dimension of the chopper, as the size of the light beam and its relative position to the chopper change with samples, the optimum phase of the signal for synchronous rectification changes.
The signal for synchronous rectification is obtained by detecting the rotation of the chopper by means of a detector such as a photocoupler, and the phase of the synchronous rectification signal is adjusted by mechanically adjusting the position of the photocoupler relative to the chopper.
Suppose that a light beam defined by a relatively large sample has a cross-sectional area and a position relative to a chopper 2 as shown at 4 in FIG. 4, and that a light beam defined by a relatively small sample has a cross-sectional area and a position relative to the chopper as shown at 6 in FIG. 4. If the signal for synchronous rectification has an optimum phase for the light beam 4 as shown at A in FIG. 5, a signal as shown at B in FIG. 5 is obtained upon synchronous rectification of the signal caused by the light beam 4 with the synchronous rectification signal A. If the signal caused by the light beam 6 is rectified with the same synchronous rectification signal A, however, a rectified signal as shown at C in FIG. 5 is obtained. The signal C is detected as an apparently smaller signal, with resulting deterioration of the signal-to-noise (S/N) ratio provided that the noise caused by the detector and other elements remains unchanged.
In the prior art arrangement, as the position of the detector such as a photocoupler for detecting the rotation of the modulating means or the rotating chopper is fixed after adjustment of the detector, the phase of the synchronous rectification signal is also fixed, so that it is impossible to readjust the phase of the synchronous rectification signal when the light beam has changed as shown in FIG. 4.